Since AutoCAD 2005 (if I recall correctly), PURGE also supports the removal of Registered Application names. Applications that make use of Extended Entity Data (XData) must register unique application names in drawings. A few applications have, in the past, created many more names than they required, and as these RegApp names get copied from drawing to drawing (when they get XRefed, for instance), they ended spreading from DWG file to DWG file (one comparison I remember hearing was to a virus, although that was perhaps a little extreme). To the best of my knowledge the applications that mistakenly caused this problem have now been fixed (and shall remain nameless), but there are still drawings out there with a lot of these records, which is ultimately why we extended PURGE to address the problem from within AutoCAD.

Anyway, I've chosen to implement a command to purge these RegApp names - even though it's now there in the product - really just an example of how to code this type of functionality. It could very easily be extended to handle the specific data you feel needs purging in your company's (or customers') DWG files.

The Database.Purge() function available in .NET wraps AcDbDatabase::purge(). These functions are both misleadingly named, as neither of them actually performs a purge on the database. What they do - and this is actually more useful, as it gives you more control - is filter a list of object IDs you pass in, removing any that cannot safely be purged by your application. So they would more accurately be named TellMeWhatCanSafelyBePurged(), or something like that. Typically objects get removed from the list because a reference exists somewhere in the drawing to that object - such as from an entity to a layer (making the layer dangerous to purge). The code calling the Purge function will usually then erase the objects that have been returned. And that's how the PURGE command is implemented.

Here's some C# code that purges the Registered Application names in the current document: